Concepts in restoration ecology - Restoration Ecology - page 25

even without an indication of which type of function
is associated with the species group (see Woodward
& Cramer 1996, Gitay & Noble 1997). This holds
strongly for inductive approaches, in which it is taken
for granted that functional groups exist and multivariate
techniques are used to seek clusters of species; the
search for syndromes fits within this approach (see sev-
eral examples in Lavorel & Cramer 1999). Deductive
methods are based on the a priori statement of the
importance of particular processes or properties in the
functioning of an ecosystem; a feasible set of func-
tional categories (e.g. annual or perennial species, C 3
or C 4 grasses, nitrogen-fixing Leguminosae) is then
deduced from these premises. If a shared class of
resources is used as the criterion, then the functional
group is a guild (Root 1967). The species assigned
to a guild need not be taxonomically related, and a
species can be member of more than one guild (Calow
1998).
Removal experiments carried out by Wardle et al .
(1999) over a 3-year period have shown that removal
of plant functional groups can have important effects
on the composition of the remainder of the flora, and
that this can influence vegetation dynamics, biomass,
productivity and diversity. They provided evidence indic-
ating that above-ground responses have the potential
to induce corresponding below-ground responses,
affecting soil food webs, community composition of
soil organisms and their diversity, and ultimately
ecosystem properties and processes. This type of ana-
lysis is derived from the efforts to identify keystone
species. The term was coined by Paine (1966) and has
been used rather loosely by very many authors there-
after. Keystone species are defined by having a key
function in controlling the structure of a food web
or the functioning of an ecosystem, which can be
identified by removal of the species from the system.
The main information on keystone species has been
derived from removal experiments (Paine 1980,
Wardle et al . 1999), from exclosure experiments
(Drent & Prins 1987, Drent & van der Wal 1999) and
by estimating interaction strengths (Menge et al .
1994). Keystone species can be identified among
plants, herbivores, predators, parasites and all other com-
ponents of a biocoenosis. Tanner and Hughes (1994),
working on coral reefs, showed that the importance
of a species to the dynamics of an assemblage may
be unrelated to its abundance at equilibrium. Note,

therefore, that rare species groups can have a greater
impact than more common ones.
Apart from intended and scientifically designed
field experiments, incidental natural disturbances
have also provided insight into the role of particular
species in their respective ecosystems. For example,
some authors suggested that a sudden bush en-
croachment in African savannas was probably caused
by an outbreak of anthrax and rinderpest among
impalas, leading to a very high survival of saplings
of acacia trees (Prins & van der Jeugd 1993, Sinclair
1995). Similarly, outbreaks of myxomatosis among
rabbit populations resulted in succession of grassland
vegetation due to the establishment of seedling trees;
for example the recruitment of oaks in England fol-
lowing rabbit myxomatosis (Crawley 1983). Lesser
snow geese, ever increasing in population size, over-
exploited the tundra vegetation in the Hudson Bay
salt marshes, which led to irreversible degeneration
of tundra vegetation (Jefferies 1999). These examples
illustrate that the fate of keystone species (be it a popu-
lation collapse or a dramatic increase in population
size) may have far-reaching consequences for other
components of a food web. This phenomenon is often
labelled 'trophic cascade'. This term has its origin in
Paine's work in marine intertidal systems (Paine
1980) and has been applied to several other types of
ecosystem where primary producers and herbivores
interact (e.g. Carpenter et al . 1985, Jefferies 1999). The
literature on this topic was reviewed by Persson
(1999), who defined a trophic cascade as the pro-
pagation of indirect mutualism between non-adjacent
levels in a food chain, but the term has also been used
to include horizontal interactions (see also Chap-
ters 4 and 6).

Indicator species

In restoration projects, indicators can be very usefully
applied to characterizing the status of success. While
keystone species play a relatively central role in the
functioning of a food web or an ecosystem, this is in
general not the case for indicator species, which can
even be a single individual of a species. Indicators are
supposed to be or have been proven to be indicative
of certain abiotic ecosystem conditions. In cases of a
single environmental factor (e.g. wetness characteristics
of a mire vegetation; Tüxen 1954), the relationship is